Online anonymity brings out the worst in people. Wilfully slanderous allegations can reach a massive audience and they encourage others to weigh in with abuse of their own.
Sometimes it seems, nobody is safe from the baying mob.
"At first I took it as just a groundless rumor posted by an Internet addict," said comedian Smiley Kikuchi, 43, who was accused of involvement in a murder in 1999. He suffered a deluge of hate-filled comments on his blog and elsewhere.
Kikuchi said what followed was a painful decade. Although groundless, the allegations were routinely repeated as fact and people came to consider him a convicted murderer.
Among threats he received, one message said his family would be attacked.
He summoned help from the police. In 2009, they turned over to prosecutors a case against seven Internet users for sending abusive comments, but the case was later dismissed.
"I was surprised to find that all of them were ordinary office workers," Kikuchi said. "They claimed they had not meant harm and had spread the rumor out of a sense of righteousness."
Kikuchi added that abuse like this harms one person most of all: the victim.
He now tries to warn young people about danger on the Internet and the impact it can have on ordinary peoples' lives. The speed of how it can happen is particularly alarming, given that one message on Twitter can hurl itself around the world in no time.
"People who retweet messages containing abuse are no different from those who post them," Kikuchi said.